The Haripin deletion (Thp) in chromosome 17 of the mouse offers the exciting prospect of uncovering preferential gene expression during embryogenesis. Interestingly, if the Thp chromosome is inherited from the mother, embryos die at about mid-gestation, but paternal heterozygotes are normal, except for their shortened tails. Homozygotes arrest as abnormal blastocysts. Nuclear transplantations between zygotes establish that the maternal effect results from the embryonic genome and not oocyte cytoplasm. Normal development apparently requires a normal chromosome 17 via the female gamete. A new recombinant chromosome 17 (twLub2) carries a small deletion in the Thp region. It too shows a maternal effect: embryos resemble maternal Thp heterozygotes. The Fused (Fu) mutation, mapping to the same chromosomal region, shows low ratios of heterozygotes born of heterozygous mothers. Two alleles of Fu, Knobbly (FuKb) and Kink (FuKi), show normal ratios of heterozygous progeny, but each is lethal in homozygous embryos around 8 days of gestation. This research has the primary aim of defining tissue and cell defects in maternal heterozygoes as well as the time of onset. Progeny tests, histological and ultrastructural investigations are planned: on maternal heterozygotes for Thp, twLub2, and Fu; on homozygotes for Thp and for twLub2; on the compounds Thp/twLub2, FuKb/Thp, and FuKb/twLub2. Observations should reveal precise cellular defects, and whether effects of the two deletions and Fu mutations are similar. These findings may resolve the effects to the preferential activity of a single "maternal effect" locus. Genetic maternal effects due to an autosome have been recognized only rarely in mammals. Unknown causes of developmental arrest in other species, including human, may have their root in a maternal regulation similar to these deletions and mutations. The Hairpin, Lub2, and Fused effects therefore may prove to have broad implications for understanding controls of embryogenesis.